1Building External Modules
2 3This document describes how to build an out-of-tree kernel module.
4 5=== Table of Contents
6 7 === 1 Introduction
8 === 2 How to Build External Modules
9 --- 2.1 Command Syntax
10 --- 2.2 Options
11 --- 2.3 Targets
12 --- 2.4 Building Separate Files
13 === 3. Creating a Kbuild File for an External Module
14 --- 3.1 Shared Makefile
15 --- 3.2 Separate Kbuild file and Makefile
16 --- 3.3 Binary Blobs
17 --- 3.4 Building Multiple Modules
18 === 4. Include Files
19 --- 4.1 Kernel Includes
20 --- 4.2 Single Subdirectory
21 --- 4.3 Several Subdirectories
22 === 5. Module Installation
23 --- 5.1 INSTALL_MOD_PATH
24 --- 5.2 INSTALL_MOD_DIR
25 === 6. Module Versioning
26 --- 6.1 Symbols From the Kernel (vmlinux + modules)
27 --- 6.2 Symbols and External Modules
28 --- 6.3 Symbols From Another External Module
29 === 7. Tips & Tricks
30 --- 7.1 Testing for CONFIG_FOO_BAR
31 32 33 34=== 1. Introduction
35 36"kbuild" is the build system used by the Linux kernel. Modules must use
37kbuild to stay compatible with changes in the build infrastructure and
38to pick up the right flags to "gcc." Functionality for building modules
39both in-tree and out-of-tree is provided. The method for building
40either is similar, and all modules are initially developed and built
41out-of-tree.
42 43Covered in this document is information aimed at developers interested
44in building out-of-tree (or "external") modules. The author of an
45external module should supply a makefile that hides most of the
46complexity, so one only has to type "make" to build the module. This is
47easily accomplished, and a complete example will be presented in
48section 3.
49 50 51=== 2. How to Build External Modules
52 53To build external modules, you must have a prebuilt kernel available
54that contains the configuration and header files used in the build.
55Also, the kernel must have been built with modules enabled. If you are
56using a distribution kernel, there will be a package for the kernel you
57are running provided by your distribution.
58 59An alternative is to use the "make" target "modules_prepare." This will
60make sure the kernel contains the information required. The target
61exists solely as a simple way to prepare a kernel source tree for
62building external modules.
63 64NOTE: "modules_prepare" will not build Module.symvers even if
65CONFIG_MODVERSIONS is set; therefore, a full kernel build needs to be
66executed to make module versioning work.
67 68--- 2.1 Command Syntax
69 70 The command to build an external module is:
71 72 $ make -C <path_to_kernel_src> M=$PWD
73 74 The kbuild system knows that an external module is being built
75 due to the "M=<dir>" option given in the command.
76 77 To build against the running kernel use:
78 79 $ make -C /lib/modules/`uname -r`/build M=$PWD
80 81 Then to install the module(s) just built, add the target
82 "modules_install" to the command:
83 84 $ make -C /lib/modules/`uname -r`/build M=$PWD modules_install
85 86--- 2.2 Options
87 88 ($KDIR refers to the path of the kernel source directory.)
89 90 make -C $KDIR M=$PWD
91 92 -C $KDIR
93 The directory where the kernel source is located.
94 "make" will actually change to the specified directory
95 when executing and will change back when finished.
96 97 M=$PWD
98 Informs kbuild that an external module is being built.
99 The value given to "M" is the absolute path of the
100 directory where the external module (kbuild file) is
101 located.
102 103--- 2.3 Targets
104 105 When building an external module, only a subset of the "make"
106 targets are available.
107 108 make -C $KDIR M=$PWD [target]
109 110 The default will build the module(s) located in the current
111 directory, so a target does not need to be specified. All
112 output files will also be generated in this directory. No
113 attempts are made to update the kernel source, and it is a
114 precondition that a successful "make" has been executed for the
115 kernel.
116 117 modules
118 The default target for external modules. It has the
119 same functionality as if no target was specified. See
120 description above.
121 122 modules_install
123 Install the external module(s). The default location is
124 /lib/modules/<kernel_release>/extra/, but a prefix may
125 be added with INSTALL_MOD_PATH (discussed in section 5).
126 127 clean
128 Remove all generated files in the module directory only.
129 130 help
131 List the available targets for external modules.
132 133--- 2.4 Building Separate Files
134 135 It is possible to build single files that are part of a module.
136 This works equally well for the kernel, a module, and even for
137 external modules.
138 139 Example (The module foo.ko, consist of bar.o and baz.o):
140 make -C $KDIR M=$PWD bar.lst
141 make -C $KDIR M=$PWD baz.o
142 make -C $KDIR M=$PWD foo.ko
143 make -C $KDIR M=$PWD /
144 145 146=== 3. Creating a Kbuild File for an External Module
147 148In the last section we saw the command to build a module for the
149running kernel. The module is not actually built, however, because a
150build file is required. Contained in this file will be the name of
151the module(s) being built, along with the list of requisite source
152files. The file may be as simple as a single line:
153 154 obj-m := <module_name>.o
155 156The kbuild system will build <module_name>.o from <module_name>.c,
157and, after linking, will result in the kernel module <module_name>.ko.
158The above line can be put in either a "Kbuild" file or a "Makefile."
159When the module is built from multiple sources, an additional line is
160needed listing the files:
161 162 <module_name>-y := <src1>.o <src2>.o ...
163 164NOTE: Further documentation describing the syntax used by kbuild is
165located in Documentation/kbuild/makefiles.txt.
166 167The examples below demonstrate how to create a build file for the
168module 8123.ko, which is built from the following files:
169 170 8123_if.c
171 8123_if.h
172 8123_pci.c
173 8123_bin.o_shipped <= Binary blob
174 175--- 3.1 Shared Makefile
176 177 An external module always includes a wrapper makefile that
178 supports building the module using "make" with no arguments.
179 This target is not used by kbuild; it is only for convenience.
180 Additional functionality, such as test targets, can be included
181 but should be filtered out from kbuild due to possible name
182 clashes.
183 184 Example 1:
185 --> filename: Makefile
186 ifneq ($(KERNELRELEASE),)
187 # kbuild part of makefile
188 obj-m := 8123.o
189 8123-y := 8123_if.o 8123_pci.o 8123_bin.o
190 191 else
192 # normal makefile
193 KDIR ?= /lib/modules/`uname -r`/build
194 195 default:
196 $(MAKE) -C $(KDIR) M=$$PWD
197 198 # Module specific targets
199 genbin:
200 echo "X" > 8123_bin.o_shipped
201 202 endif
203 204 The check for KERNELRELEASE is used to separate the two parts
205 of the makefile. In the example, kbuild will only see the two
206 assignments, whereas "make" will see everything except these
207 two assignments. This is due to two passes made on the file:
208 the first pass is by the "make" instance run on the command
209 line; the second pass is by the kbuild system, which is
210 initiated by the parameterized "make" in the default target.
211 212--- 3.2 Separate Kbuild File and Makefile
213 214 In newer versions of the kernel, kbuild will first look for a
215 file named "Kbuild," and only if that is not found, will it
216 then look for a makefile. Utilizing a "Kbuild" file allows us
217 to split up the makefile from example 1 into two files:
218 219 Example 2:
220 --> filename: Kbuild
221 obj-m := 8123.o
222 8123-y := 8123_if.o 8123_pci.o 8123_bin.o
223 224 --> filename: Makefile
225 KDIR ?= /lib/modules/`uname -r`/build
226 227 default:
228 $(MAKE) -C $(KDIR) M=$$PWD
229 230 # Module specific targets
231 genbin:
232 echo "X" > 8123_bin.o_shipped
233 234 The split in example 2 is questionable due to the simplicity of
235 each file; however, some external modules use makefiles
236 consisting of several hundred lines, and here it really pays
237 off to separate the kbuild part from the rest.
238 239 The next example shows a backward compatible version.
240 241 Example 3:
242 --> filename: Kbuild
243 obj-m := 8123.o
244 8123-y := 8123_if.o 8123_pci.o 8123_bin.o
245 246 --> filename: Makefile
247 ifneq ($(KERNELRELEASE),)
248 # kbuild part of makefile
249 include Kbuild
250 251 else
252 # normal makefile
253 KDIR ?= /lib/modules/`uname -r`/build
254 255 default:
256 $(MAKE) -C $(KDIR) M=$$PWD
257 258 # Module specific targets
259 genbin:
260 echo "X" > 8123_bin.o_shipped
261 262 endif
263 264 Here the "Kbuild" file is included from the makefile. This
265 allows an older version of kbuild, which only knows of
266 makefiles, to be used when the "make" and kbuild parts are
267 split into separate files.
268 269--- 3.3 Binary Blobs
270 271 Some external modules need to include an object file as a blob.
272 kbuild has support for this, but requires the blob file to be
273 named <filename>_shipped. When the kbuild rules kick in, a copy
274 of <filename>_shipped is created with _shipped stripped off,
275 giving us <filename>. This shortened filename can be used in
276 the assignment to the module.
277 278 Throughout this section, 8123_bin.o_shipped has been used to
279 build the kernel module 8123.ko; it has been included as
280 8123_bin.o.
281 282 8123-y := 8123_if.o 8123_pci.o 8123_bin.o
283 284 Although there is no distinction between the ordinary source
285 files and the binary file, kbuild will pick up different rules
286 when creating the object file for the module.
287 288--- 3.4 Building Multiple Modules
289 290 kbuild supports building multiple modules with a single build
291 file. For example, if you wanted to build two modules, foo.ko
292 and bar.ko, the kbuild lines would be:
293 294 obj-m := foo.o bar.o
295 foo-y := <foo_srcs>
296 bar-y := <bar_srcs>
297 298 It is that simple!
299 300 301=== 4. Include Files
302 303Within the kernel, header files are kept in standard locations
304according to the following rule:
305 306 * If the header file only describes the internal interface of a
307 module, then the file is placed in the same directory as the
308 source files.
309 * If the header file describes an interface used by other parts
310 of the kernel that are located in different directories, then
311 the file is placed in include/linux/.
312 313 NOTE: There are two notable exceptions to this rule: larger
314 subsystems have their own directory under include/, such as
315 include/scsi; and architecture specific headers are located
316 under arch/$(ARCH)/include/.
317 318--- 4.1 Kernel Includes
319 320 To include a header file located under include/linux/, simply
321 use:
322 323 #include <linux/module.h>
324 325 kbuild will add options to "gcc" so the relevant directories
326 are searched.
327 328--- 4.2 Single Subdirectory
329 330 External modules tend to place header files in a separate
331 include/ directory where their source is located, although this
332 is not the usual kernel style. To inform kbuild of the
333 directory, use either ccflags-y or CFLAGS_<filename>.o.
334 335 Using the example from section 3, if we moved 8123_if.h to a
336 subdirectory named include, the resulting kbuild file would
337 look like:
338 339 --> filename: Kbuild
340 obj-m := 8123.o
341 342 ccflags-y := -Iinclude
343 8123-y := 8123_if.o 8123_pci.o 8123_bin.o
344 345 Note that in the assignment there is no space between -I and
346 the path. This is a limitation of kbuild: there must be no
347 space present.
348 349--- 4.3 Several Subdirectories
350 351 kbuild can handle files that are spread over several directories.
352 Consider the following example:
353 354 .
355 |__ src
356 | |__ complex_main.c
357 | |__ hal
358 | |__ hardwareif.c
359 | |__ include
360 | |__ hardwareif.h
361 |__ include
362 |__ complex.h
363 364 To build the module complex.ko, we then need the following
365 kbuild file:
366 367 --> filename: Kbuild
368 obj-m := complex.o
369 complex-y := src/complex_main.o
370 complex-y += src/hal/hardwareif.o
371 372 ccflags-y := -I$(src)/include
373 ccflags-y += -I$(src)/src/hal/include
374 375 As you can see, kbuild knows how to handle object files located
376 in other directories. The trick is to specify the directory
377 relative to the kbuild file's location. That being said, this
378 is NOT recommended practice.
379 380 For the header files, kbuild must be explicitly told where to
381 look. When kbuild executes, the current directory is always the
382 root of the kernel tree (the argument to "-C") and therefore an
383 absolute path is needed. $(src) provides the absolute path by
384 pointing to the directory where the currently executing kbuild
385 file is located.
386 387 388=== 5. Module Installation
389 390Modules which are included in the kernel are installed in the
391directory:
392 393 /lib/modules/$(KERNELRELEASE)/kernel/
394 395And external modules are installed in:
396 397 /lib/modules/$(KERNELRELEASE)/extra/
398 399--- 5.1 INSTALL_MOD_PATH
400 401 Above are the default directories but as always some level of
402 customization is possible. A prefix can be added to the
403 installation path using the variable INSTALL_MOD_PATH:
404 405 $ make INSTALL_MOD_PATH=/frodo modules_install
406 => Install dir: /frodo/lib/modules/$(KERNELRELEASE)/kernel/
407 408 INSTALL_MOD_PATH may be set as an ordinary shell variable or,
409 as shown above, can be specified on the command line when
410 calling "make." This has effect when installing both in-tree
411 and out-of-tree modules.
412 413--- 5.2 INSTALL_MOD_DIR
414 415 External modules are by default installed to a directory under
416 /lib/modules/$(KERNELRELEASE)/extra/, but you may wish to
417 locate modules for a specific functionality in a separate
418 directory. For this purpose, use INSTALL_MOD_DIR to specify an
419 alternative name to "extra."
420 421 $ make INSTALL_MOD_DIR=gandalf -C $KDIR \
422 M=$PWD modules_install
423 => Install dir: /lib/modules/$(KERNELRELEASE)/gandalf/
424 425 426=== 6. Module Versioning
427 428Module versioning is enabled by the CONFIG_MODVERSIONS tag, and is used
429as a simple ABI consistency check. A CRC value of the full prototype
430for an exported symbol is created. When a module is loaded/used, the
431CRC values contained in the kernel are compared with similar values in
432the module; if they are not equal, the kernel refuses to load the
433module.
434 435Module.symvers contains a list of all exported symbols from a kernel
436build.
437 438--- 6.1 Symbols From the Kernel (vmlinux + modules)
439 440 During a kernel build, a file named Module.symvers will be
441 generated. Module.symvers contains all exported symbols from
442 the kernel and compiled modules. For each symbol, the
443 corresponding CRC value is also stored.
444 445 The syntax of the Module.symvers file is:
446 <CRC> <Symbol> <module>
447 448 0x2d036834 scsi_remove_host drivers/scsi/scsi_mod
449 450 For a kernel build without CONFIG_MODVERSIONS enabled, the CRC
451 would read 0x00000000.
452 453 Module.symvers serves two purposes:
454 1) It lists all exported symbols from vmlinux and all modules.
455 2) It lists the CRC if CONFIG_MODVERSIONS is enabled.
456 457--- 6.2 Symbols and External Modules
458 459 When building an external module, the build system needs access
460 to the symbols from the kernel to check if all external symbols
461 are defined. This is done in the MODPOST step. modpost obtains
462 the symbols by reading Module.symvers from the kernel source
463 tree. If a Module.symvers file is present in the directory
464 where the external module is being built, this file will be
465 read too. During the MODPOST step, a new Module.symvers file
466 will be written containing all exported symbols that were not
467 defined in the kernel.
468 469--- 6.3 Symbols From Another External Module
470 471 Sometimes, an external module uses exported symbols from
472 another external module. kbuild needs to have full knowledge of
473 all symbols to avoid spitting out warnings about undefined
474 symbols. Three solutions exist for this situation.
475 476 NOTE: The method with a top-level kbuild file is recommended
477 but may be impractical in certain situations.
478 479 Use a top-level kbuild file
480 If you have two modules, foo.ko and bar.ko, where
481 foo.ko needs symbols from bar.ko, you can use a
482 common top-level kbuild file so both modules are
483 compiled in the same build. Consider the following
484 directory layout:
485 486 ./foo/ <= contains foo.ko
487 ./bar/ <= contains bar.ko
488 489 The top-level kbuild file would then look like:
490 491 #./Kbuild (or ./Makefile):
492 obj-y := foo/ bar/
493 494 And executing
495 496 $ make -C $KDIR M=$PWD
497 498 will then do the expected and compile both modules with
499 full knowledge of symbols from either module.
500 501 Use an extra Module.symvers file
502 When an external module is built, a Module.symvers file
503 is generated containing all exported symbols which are
504 not defined in the kernel. To get access to symbols
505 from bar.ko, copy the Module.symvers file from the
506 compilation of bar.ko to the directory where foo.ko is
507 built. During the module build, kbuild will read the
508 Module.symvers file in the directory of the external
509 module, and when the build is finished, a new
510 Module.symvers file is created containing the sum of
511 all symbols defined and not part of the kernel.
512 513 Use "make" variable KBUILD_EXTRA_SYMBOLS
514 If it is impractical to copy Module.symvers from
515 another module, you can assign a space separated list
516 of files to KBUILD_EXTRA_SYMBOLS in your build file.
517 These files will be loaded by modpost during the
518 initialization of its symbol tables.
519 520 521=== 7. Tips & Tricks
522 523--- 7.1 Testing for CONFIG_FOO_BAR
524 525 Modules often need to check for certain CONFIG_ options to
526 decide if a specific feature is included in the module. In
527 kbuild this is done by referencing the CONFIG_ variable
528 directly.
529 530 #fs/ext2/Makefile
531 obj-$(CONFIG_EXT2_FS) += ext2.o
532 533 ext2-y := balloc.o bitmap.o dir.o
534 ext2-$(CONFIG_EXT2_FS_XATTR) += xattr.o
535 536 External modules have traditionally used "grep" to check for
537 specific CONFIG_ settings directly in .config. This usage is
538 broken. As introduced before, external modules should use
539 kbuild for building and can therefore use the same methods as
540 in-tree modules when testing for CONFIG_ definitions.
541 542